Electromagnetic inverse scattering is one of the most effective approaches for imaging and estimating the permittivity of unknown scatterers.
Electromagnetic inverse scattering has many applications in the pharmaceutical industry, such as pharmaceutical industry quality control. Other exemplary areas of use include medical imaging, cancer diagnosis and treatment, remote sensing, radio-astronomy, industrial quality control, security, and defence. The spatial permittivity or permittivity profile estimation of objects has been investigated widely using volume equivalent current source (VECS) formulation and solving the electromagnetic inverse scattering equation. In general, existing methods for permittivity profile estimation can be categorized into three main approaches radiating, non-radiating, and noise VECS. Electromagnetic Scattering Tomography system is an inspiring alternative to existing imaging modalities. The need for an alternative imaging system to complement the existing medical imaging modalities is undeniable in today's medical health care practice. The biological tissue screening and examinations are done by means of magnetic resonance, computed tomography (CT), x-ray, and ultrasound imaging modalities. Magnetic resonance images (MRIs) are good for imaging the soft tissues but not hard tissues (i.e. bones or teeth). CTs are used for imaging and diagnosis of the hard tissues in practice but not soft tissues. Ultrasound cannot be used for imaging inside the hard tissues but can be used for imaging the outer surface of hard tissues. Since all parts in a living biosystem are dielectric in nature, they can be considered as lossy dielectric and electromagnetic waves can penetrate inside the biosystem. For that reason, several electromagnetic imaging systems were developed to meet the high demand for an alternative imaging modality for clinical applications.
Depending on the shape of the observation domain, the electromagnetic scattering based imaging systems are classified into two main categories: planar imaging and non-planar imaging. A brief review of the existing planar electromagnetic scattering systems reported in literature is presented below.
A planar electromagnetic scattering system for coherent projection imaging has been reported. In some systems, the planar electromagnetic scattering computerized tomography was developed without considering diffraction or scattering phenomenon. Since they assumed that electromagnetic fields propagate through a scatterer in a straight line, the transmitting and receiving antennas were moved around the stationary scatterer over 180°, similar to CT. This is not a valid assumption for electromagnetic scattering imaging system. Later on, for faster scanning speeds, the electronic raster scanning of the electric fields was considered. Generally speaking, the electronic raster scanning based tomographic systems suffer from a few drawbacks: 1) the measurement fields are disturbed by the antenna elements placed adjacent to the receiving antenna element and 2) the receiving antenna elements are cross-talked.
The electromagnetic scattering tomography systems are implemented in an Anechoic chamber to control the multipath effects. Since using an Anechoic chamber is not an option for clinical applications, water has been widely used as the background medium for the electro-magnetic scattering imaging system for reducing the multipath effects and lowering the contrast of the scatterer. To have images with higher resolution, higher frequency is recommended; higher frequencies increase the electric field attenuation in water.